1. Field of the Invention
This invention relates generally to switching power converters, and more particularly, to a sensorless current mode control method for such a converter.
2. Description of the Related Art
Switching power converters require feedback to regulate their output, and as such they typically require frequency compensation in order to guarantee stability. To this end, various switching power converter controller architectures have been developed with different compensation requirements, which provide different levels of performance with respect to stability and transient response.
Three of the most common controller architectures are categorized as follows:                1) Voltage mode        2) Voltage mode with input voltage feed-forward        3) Current mode (with voltage mode)Each has advantages and disadvantages. Voltage mode is relatively easy to implement, but harder to compensate and typically does not provide as good a transient performance as current mode. Voltage mode with input voltage feed-forward provides better input line rejection, albeit not as good as current mode, as the instantaneous correction provided by input voltage feed-forward is not an exact equivalent for the integrating nature of an inductor.        
Although current mode controllers arguably provide the best overall performance, practical implementations of current mode can be troublesome and performance often falls short of theoretical. Ironically, the difficulties in implementing current mode typically preclude its use in higher frequency switching regulators. Although voltage mode controllers have a slower transient response, they are generally easier to operate at higher frequencies. Most high-frequency switching regulators on the market therefore employ voltage mode control. This results in these higher frequency voltage mode switching regulators ironically having slower transient responses than lower frequency current mode solutions.
Current mode controller performance must be traded-off with efficiency, since some series impedance is needed in the current path in order to sense current. For efficiency reasons, the voltage drop across this impedance is often designed to be very low, and hence this voltage signal is subject to significant noise. Another problem with current mode is settling time. When a switching converter's power switch is ‘off’, the voltage drop across it is very large. After the switch turns on, this voltage drops from very large to a very small value extremely rapidly. Inductive and capacitive stray effects can cause significant glitches immediately following this transition. These factors make current sensing in a switch difficult, due to the settling time needed. As an example, an ‘off’ switch may have 5V across it, and the same switch turned on with 0.5 amp flowing through it may only have 0.1 v of drop across it—and this is the full-scale signal. Usually a small fraction of the full-scale signal needs to be measured—a typical number being 0.02V.
Another issue with current mode control is that it typically requires an increase in the minimum operating voltage. This is because internally, the load current is replicated by a voltage signal. This replica signal rises and falls with the load current, therefore requiring a higher operating headroom voltage than would a voltage-mode controller. A converter that runs on a 3-5V supply voltage may need 0.5V of internal signal range for the replica of the inductor or switch current.
In order to mitigate some of these issues, a method called ‘emulated current mode’ is often used, typically in high duty ratio conditions where the current signal needs to be sensed during the shorter of the two switching periods. For example, in a step-down converter where peak current in the high-side switch is monitored and the input voltage is much larger than the output voltage, the on-time in the high-side switch is very short. In this example, emulated current mode would actually sense the true current during the low side conduction instead, when the inductor current is ramping down and the duration of the low-side on-time is long. This sensed signal is replicated as a proportional voltage onto a capacitor. At the end of the low-side on-time, this capacitor now contains the DC information about the inductor current. When the high-side switch turns on, instead of trying to measure the high-side switch current, the capacitor is charged by a current proportional to the inductor drive voltage. Thus, the capacitor voltage now emulates the high-side current, including the DC information, during the high-side on-time. The advantages of this over true current sensing are twofold: first, no settling time is needed, and second, because the capacitor is an integrator, it is by nature a noise filter. Also, any slight DC error during the emulation period is immediately removed on the next low-side on-time, so that DC errors do not accumulate on the capacitor.